Uh. Wow. I’m crap at giving advice. XD But, uhhh, I guess I could give it a shot. :P
Okay… uh… well, as a hardcore SPN fan, I kinda have to tell you you must watch everything. I mean, there are a lot of filler episodes, but there’s kind of always something crucial in them for the whole story.
And, I mean, even if you could skip some, I think it’d be better for you to just watch everything, cause then you get to see all of the things and make inside jokes with the rest of the fandom [trust me. Inside jokes of this fandom = awesome.] and… stuff. It took me like forever and a day to watch all the episodes (I missed two of them the first time round, and that screwed me up majorly because they were the last two eps of a season) too, but it’s so worth it. That could just be the fangirl in me speaking, but I really think it is.
So, uhmmm, tl;dr, you should watch everything. It’ll take a while, and I’m getting that weird professional tone again, but it’s worth it. Seriously.
In other news, however: FOLLOWERS HELP THIS PERSON BECAUSE I’M USELESS.